1. Field of the Invention
The present invention relates to a method of preparing a platinum alloy electrode catalyst for a direct methanol fuel cell (DMFC) using anhydrous metal chlorides and, more particularly, to a method of preparing a platinum alloy electrode catalyst for DMFC with minimized particle size from platinum chloride and a second metal chloride using a boron lithium hydride (LiBH4) reductant and an organic water-incompatible solvent.
2. Description of the Related Art
DMFC is a non-pollutive energy source operable at a low temperature of below 100xc2x0 C. and useful in a variety of applications, including power sources of non-pollutive vehicles, local generating and moving power source, etc. However, DMFC has a problem in that carbon monoxide (CO) generated as a by-product from oxidation of methanol causes a poisoning effect to the platinum used as an electrode catalyst of the fuel cell, thus deteriorating the oxidation performance of the methanol.
In an attempt to overcome the problem, many studies have been made on using a platinum-based alloy whose particle size is reduced with a small amount rather than pure platinum so as to minimize the poisoning effect to the platinum caused by carbon monoxide and achieve economical gains.
In regard to this, the conventional precipitation method for preparing an electrode catalyst for DMFC involves addition of various reductants (e.g., sodium formate, sodium thiosulfite, or nitrohydrazine) to an aqueous solution of metal chloride to form nano-order particles, which method is relatively simple in procedure but disadvantageous in regard to necessity of using water-soluble metal chlorides exclusively (References: N. M. Kagan, Y. N. Pisarev, Y. A. Kaller, V. A. Panchenko, Elektrokhimiya, 9, 1498 (1973); and C. N. Glavee, K. N. Klabunde, C. M. Sorensen, G. C. Hajipanayis, Langmuir, 9, 162 (1993)).
Another method is known that involves dispersion of a non-aqueous metal chloride in water together with carbon, drying the dispersed metal chloride, and then reduction of the metal chloride via heat treatment in the hydrogen atmosphere, which method has a necessity of heat treatment at a high temperature with a consequence of an increase in the particle size due to heat treatment, thus providing non-uniform particle size distribution (Reference: T. A. Dorling, R. L. Moss, J. Catal., 5, 111 (1966)).
Further, a colloid method of preparing an electrode catalyst for DMFC in an aqueous solution has been proposed (Reference: Watanabe et al., Journal of Electronal. Chem., 229, 395 (1987)). This method is a multi-step procedure involving addition of a reductant (e.g., Na2CO3 or NaHSO4) to a hydrated platinum chloride (H2PtCl6.xH2O) and an aqueous solution of a second metal chloride to form a metal composite intermediate, hydrolysis of the intermediate to form a metal oxide, and then heat treatment in the hydrogen atmosphere to produce nano-sized platinum alloy particles. However, the yield of the platinum alloy is significantly dependent upon controlling the types of reductants added in the respective steps and the change in the acidity (pH) between the steps. This method is also disadvantageous in regard to necessity of heat treatment in the hydrogen atmosphere in order to complete reduction and catalytic activity of the metal oxide.
Another colloid method of preparing an electrode catalyst for DMFC on a non-aqueous solution has been proposed (Reference: Bonnemann et al., Angew. Chem. Int. Ed. Engl., 30(10), 1312 (1991)), which involves preparing a surfactant-stabilized catalyst using a synthetic tetrabutylammonium-based reductant, oxidizing the surfactant in the oxygen atmosphere and performing a heat treatment in the hydrogen atmosphere to remove the surfactant surrounding the catalyst. This method is advantageous in regard to uniform particle size of the catalyst around 2 nm and readiness of dispersion in carbon, but problematic in that the process is too complex and the heat treatment causes agglomeration to increase the particle size.
It is, therefore, an object of the present invention to provide a method of easily preparing a platinum alloy electrode catalyst for DMFC having an average particle size of less than 2.0 nm from platinum chloride and a non-aqueous solution of a second metal chloride using a boron lithium hydride (LiBH4) reductant and an organic water-incompatible solvent without heat treatment.
To achieve the above object of the present invention, there is provided a method of preparing a platinum alloy electrode catalyst for a direct methanol fuel cell, which method includes the steps of: dissolving platinum chloride and non-aqueous second metal chloride in an organic water-incompatible solvent to prepare a platinum chloride solution and a second metal chloride solution, respectively; mixing the respective solutions to obtain a mixed solution; adding a boron lithium hydride (LiBH4) solution to the mixed solution and subjecting the mixed solution to reduction reaction to form a platinum alloy powder; and collecting the platinum alloy powder from the mixed solution through drying.